As a hydrocarbon-producing well matures, increasing water production becomes a serious problem. To reduce the production of undesired water or gas from a hydrocarbon-producing formation, aqueous polymer fluids containing crosslinking agents have been used. Such aqueous polymer fluids have been pumped into the hydrocarbon-producing formation so as to allow them to enter the water zones within and adjacent to the formations and crosslink. Such fluids have been pumped into and adjacent to water producing formations and allowed to crosslink. Crosslinking causes the aqueous polymer fluids to form a stiff gel that stops or reduces the undesired water and/or gas flow. Remediation techniques such as these are generally referred to as conformance control. Remediation techniques are selected on the basis of the water source and the method of entry into the wellbore. Conformance control treatments include sealant treatments and relative permeability modifiers (also referred to as disproportionate permeability modifiers).
A water-soluble chromium (3+) crosslinked polyacrylamide gel has been used in conformance control treatments. The gel time is usually controlled by the addition of materials that chelate with chromium in competition with the polymer-bound carboxylate groups. The crosslinking reaction in this gel system takes place by the complexation of Cr (3+) ions with carboxylate group on the polymer chain. The chemical bond between Cr (3+) and the pendant carboxylate group facilitates the formation of insoluble chromium species that can occur at high pH values. Other problems with this system include environmental concerns, thermal instability, unpredictable gel time, and gel instability in the presence of chemical species that are potential ligands.
Another water-based gel system for conformance control is based on phenol/formaldehyde crosslinker system for homo-, co-, and ter-polymer systems containing acrylamide. The crosslinking mechanism involves hydroxymethylation of the amide nitrogen, with subsequent propagation of crosslinking by multiple alkylation on the phenolic ring. Because of the nature of this chemical bond, the gel time is controllable over a wide temperature range. Although effective in gel systems, phenol and formaldehyde are highly toxic and are considered environmental hazards.
Still another water-based gel system for conformance control is based on a polyethyleneimine (PEI) crosslinker and a copolymer of acrylamide and t-butyl acrylate. PEI is a low-toxicity material that has been approved in the United States by the Food and Drug Administration for food contact. Although non-toxic, PEI may bio-accumulate or persist in the environment for long periods.
While the use of aqueous polymer fluids to reduce undesired water production has achieved varying degrees of success, these crosslinking agents are undesirable in that they either have low gel stabilities at high temperatures, have high toxicities, and/or bio-accumulate in the environment.
Thus, improved well treatment fluids and methods used to control water production and thereby enhance the production of oil or gas are needed.